Fix polygon splitting

[quchen]

Fix polygon splitting

[quchen]

62f6c8a introduced a new approach that needs improvement.

Right now, the algorithm assumes that in a cut polygon, each point can have only one outgoing line. This is wrong, since it’s one (for uncut edges) or two (for cut edges – one going in the former direction, one along the cut). The implementation arbitrarily chooses one of the two, so each cut discards one useful line.

The fix is to replace the Map Vec2 Vec2 with a Map Vec2 (Either Vec2 (Vec2,Vec2) in spirit, and handling it from there.

[quchen]

Done